1. Field of the Invention
The present invention relates generally to the art of CMOS photo detectors for acquiring images in devices such as video cameras or electronic still picture cameras, and specifically to the pixel array employed in such devices.
2. Description of the Related Art
More manufacturers have focused on using existing CMOS foundry facilities to manufacture imaging components used in image pickup devices, such as video cameras or electronic still cameras. The use of CMOS in manufacturing these devices results in relatively simple integration of the imager, signal processing circuits, and control circuitry on a single chip. The performance of CMOS photodetectors has certain shortcomings since the process was originally designed for transistors. In particular, the use of traditional CMOS technology in a photodetector device does not produce the type of performance necessary to produce high quality photo images.
Imager arrays are constructed using a single type pixel repeated over the image receiving surface. Each pixel is designed to accept, or xe2x80x9csee,xe2x80x9d a single color, such as red, green, or blue. Each pixel has a construction identical to each other pixel, i.e. no difference exists between a red pixel, a blue pixel, or a green pixel. The three different color pixel regions produce unbalanced responsivity, or different electrical responses, for each of the three different colored regions. It has been observed that systems employing identical pixel design for three different colors to construct an imager produce an excess red response, which is undesirable. Further, the use of three different pixel regions having highly similar construction can suffer from cross coupling of the electrical signals transmitted by the individual areas. Most notably, blue and green pixels can have a tendency to pick up signals from red pixels.
It has further been observed that black and white images taken in blue light appear sharper than the same image taken in red light. Further, the use of three different color pixel regions having highly similar construction can suffer from low responsiveness and high crosstalk, to name two generally perceived problems. Crosstalk is the general interference between two electromagnetic signals. Low responsiveness results in inferior signal to noise performance, while high crosstalk between the signals reduces the overall sharpness of an image.
Various methods have been used to improve the collection efficiency and overall performance of imaging devices, including utilizing different thicknesses of oxide layers to cover the red, green, and blue components of the photodiodes. This covering of the photodiode using oxide layers does not prevent the cross coupling inherent in the design, and requires exacting deposition of oxide layers which can be both expensive and time consuming. These problems diminish the quality of the image ultimately rendered.
It is therefore an object of the current invention to provide a pixelated image sensing arrangement that takes advantage of current CMOS fabrication and foundry capabilities and results in an improved quality image over previously known methods and designs.
It is a further object of the current invention to provide a CMOS image sensing pixel array design that minimizes the crosscoupling or crosstalk between signals generated by the components and also minimizes the negative effects associated with the physical characteristics inherent in the red, green, and blue nodes of the photodiode.
It is still a further object of the current invention to provide a design having increased responsiveness to light energy received over previously known designs, and particularly an enhanced collection efficiency and a balanced response for the red, green, and blue colors of the photodetector.
It is yet a further object of the current invention to provide a CMOS photodiode design having minimal crosstalk or crosscoupling between the signals of the pixel color regions, and minimize degradation of the Modulation Transfer Function.
The present invention is a color optimized CMOS photodiode pixel array that employs different dimensions to take advantage of different characteristics of the photodiode physics to produce an enhanced image that reduces the need for post processing. The design includes a relatively shallow blue pixel photodiode, a deeper green pixel photodiode, and a relatively deep red pixel photodiode, where all dimensions are relative to one another. The red pixel photodiode is responsiveness adjusted and balanced to the need of the particular arrangement, such as by resizing the components of the pixel structure. The goal is to provide a pixel suited in responsiveness to the color, such as red, intended to be received by the pixel. In one proposed embodiment, the red pixel photodiode can be deeper than the green pixel photodiode, which can be deeper than the blue pixel photodiode. Each color pixel photodiode comprises a junction diode and a depletion region sufficiently wide to provide effective responsiveness. One potential embodiment has the red junction diode having a depletion region on the order of one half the total width of the red pixel depletion region, and the depth of the red depletion region is approximately equal to twice the depth of the red junction diode. Again, the wider the depletion region, the better it is for either the red, green, or blue pixel, as a wider depletion region improves responsiveness. The blue pixel junction diode may be on the order of one third the width of the entire depletion region, while the blue pixel depletion region may be on the order of three times the depth of the blue junction diode. The red pixel junction diode may be on the order of at least three times as large as the blue pixel junction. It is understood that different dimensions may be employed while still within the scope of the current invention such that the design takes advantage of optimal responsiveness characteristics and cross coupling effects for the fabricated photodiode structure. Note that the preferred construct of the diode structures includes deposition of the depletion region on a uniform doping level layer.
The green pixel photodiode may be formed in different manners. In the present invention, the green pixel design could be any form of design between the red and blue pixel designs, preferably closer to the blue pixel design. One potential embodiment of the green pixel photodiode employs a dual stage or dual level junction diode with a dual stage depletion region. The dual stage comprises a relatively wide and shallow upper region and a narrow and deep lower region. The combination of a wide upper region and a deep lower region minimize cross coupling effects and maximize responsiveness. The depletion region in this design essentially reflects the contours of the junction diode, with a relatively large depth in the lower depletion region. An alternate embodiment of the green pixel photodiode is an intermediate construct between the blue pixel photodiode and the red pixel photodiode. The design of green pixels tends to be dependent on the pixel layout, contemplated environment, and design characteristics. In all cases it should be noted that the green pixel design should minimize cross coupling effects while maximizing responsiveness.
The CMOS construction of the three color pixel photodiodes may vary, but one possible construct of the red pixel photodiode would be an N Well/P Sub diode construct, the green pixel photodiode a N+/P Sub diode construct, and the blue being a N+/P Well diode construct. Other CMOS fabrication processes may be used for each color pixel photodiode, but these represent types of designs available for constructing the invention disclosed herein.